The invention relates to hydrophilic coatings. More specifically, the invention relates to a hydrophilic antimicrobial coating.
An environmental control system for a manned spacecraft typically includes a condensing heat exchanger for controlling cabin temperature and cabin humidity. Cabin air is drawn into the condensing heat exchanger and circulated through air passageways. Heat from the drawn-in air is transferred to a coolant, which is circulated through coolant passageways within the condensing heat exchanger. As the drawn-in air is being cooled, condensate forms on the heat transfer surfaces of the air passageways. A slurper section at an air outlet of the condensing heat exchanger removes the condensate. The dehumidified air is circulated back to the cabin, and the condensate is dumped into space or treated for reuse.
The heat transfer surfaces of the air passageways are coated with a hydrophilic coating. The hydrophilic coating, which has an affinity for attracting, adsorbing or absorbing water, promotes the condensate to form a wetting film rather than droplets within the air passageways. Water droplets, if allowed to form, can cause problems in the micro-gravity environment of space. If strongly attached to the heat transfer surfaces, the water droplets will cause an increase in air pressure drop across the condensing heat exchanger and, consequently, a reduction in performance of the condensing heat exchanger. If not strongly attached, the water drops will become entrapped in the air stream and carried into the cabin. Quite literally, rain could fall inside the cabin.
The hydrophilic coating becomes wet during the operation of the condensing heat exchanger. The moisture, in turn, provides a potential breeding ground for microbes such as bacteria and fungi. Moisture levels between 15% and 35% will predominantly support the growth of fungi, while moisture levels above 35% will also predominantly support the growth of bacteria. The microbial growths could block air passages inside the condensing heat exchanger and, consequently, reduce performance of the condensing heat exchanger. The bacteria and fungi, if allowed to proliferate, could affect the health of crew members inside the cabin.
The microbes can be killed by adding biocides to the hydrophilic coating. The biocides dissolve in water and attack the microbes in a water-solution. For example, U.S. Pat. Nos. 5,264,250 and 5,305,827 disclose the use of a biocide such as silver oxide dispersed in a glass-based multiphase hydrophilic coating made of uncoated silica and calcium silicate particles.
However, the silver oxide is intrinsically hydrophobic. When the concentration of the silver oxide exceeds 1.5% of the total weight of the glass-based multiphase hydrophilic coating, it is believed that the hydrophilicity of the hydrophilic coating (i.e., the ability of the hydrophilic coating to promote the wetting film formation) is reduced. Thus, increasing the amount of silver oxide increases the likelihood of droplets forming inside the air passageways of the heat exchanger.